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Sulfur has been observed to be severely depleted in dense clouds leading to uncertainty in the molecules that contain it and the chemistry behind their evolution. Here, we aim to shed light on the sulfur chemistry in young stellar objects (YSOs) by using high-resolution infrared spectroscopy of absorption by the ν 3 rovibrational band of SO 2 obtained with the Echelon-Cross-Echelle Spectrograph on the Stratospheric Observatory for Infrared Astronomy. Using local thermodynamic equilibrium models we derive physical parameters for the SO 2 gas in the massive YSO MonR2 IRS3. This yields a SO 2 /H abundance lower limit of 5.6 ± 0.5 × 10 -7 , or >4% of the cosmic sulfur budget, and an intrinsic line width (Doppler parameter) of b < 3.20 km s -1 . The small line widths and high temperature ( T ex = 234 ± 15 K) locate the gas in a relatively quiescent region near the YSO, presumably in the hot core where ices have evaporated. This sublimation unlocks a volatile sulfur reservoir (e.g., sulfur allotropes as detected abundantly in comet 67P/Churyumov-Gerasimenko), which is followed by SO 2 formation by warm, dense gas-phase chemistry. The narrowness of the lines makes formation of SO 2 from sulfur sputtered off grains in shocks less likely toward MonR2 IRS3.

Gas-phase chemistry in the interstellar medium is driven by fast ion-molecule reactions. This, of course, demands a mechanism for ionization, and cosmic rays are the ideal candidate as they can operate throughout the majority of both diffuse and dense interstellar clouds. Aside from driving interstellar chemistry via ionization, cosmic rays also interact with the interstellar medium in ways that heat the ambient gas, produce gamma rays, and produce light element isotopes. In this paper we review the observables generated by cosmic-ray interactions with the interstellar medium, focusing primarily on the relevance to astrochemistry.

We report observations and analysis of infrared spectra of H3(+) and CO lines in the Galactic center, within a few parsecs of the central black hole, Sgr A*. We find a cosmic ray ionization rate typically an order of magnitude higher than outside the Galactic center. Notwithstanding, the elevated cosmic ray ionization rate is 4 orders of magnitude too short to match the proton energy spectrum, as inferred from the recent discovery of the TeV γ-ray source in the vicinity of Sgr A*.

The opening of the submillimeter sky with the Herschel Space Observatory has led to the detection of new interstellar molecular ions, H2O(+), H2Cl(+), and HCl(+), which are important intermediates in the synthesis of water vapor and hydrogen chloride. In this paper, we report new observations of H2O(+) and H2Cl(+) performed with both Herschel and ground-based telescopes, to determine the abundances of their ortho and para forms separately and derive the ortho-to-para ratio. At the achieved signal-to-noise ratio, the observations are consistent with an ortho-to-para ratios of 3 for both H2O(+) and H2Cl(+), in all velocity components detected along the lines-of-sight to the massive star-forming regions W31C and W49N. We discuss the mechanisms that contribute to establishing the observed ortho-to-para ratio and point to the need for a better understanding of chemical reactions, which are important for establishing the H2O(+) and H2Cl(+) ortho-to-para ratios.

Owing to its simple chemistry, H(3)(+) is widely regarded as the most reliable tracer of the cosmic-ray ionization rate in diffuse interstellar clouds. At present, H(3)(+) observations have been made in over 50 sight lines that probe the diffuse interstellar medium (ISM) throughout the Galaxy. This small survey presents the opportunity to investigate the distribution of cosmic-ray ionization rates in the ISM, as well as any correlations between the ionization rate and line-of-sight properties. Some of the highest inferred ionization rates are about 25 times larger than the lowest upper limits, suggesting variations in the underlying low-energy cosmic-ray flux across the Galaxy. Most likely, such variations are caused predominantly by the distance between an observed cloud and the nearest site of particle acceleration.